This invention relates to improvements in high voltage gas insulated equipment, and in particular to systems for minimizing the contamination and resulting high voltage breakdown across internal support insulators.
The use of compressed gas insulated transmission lines has become more widespread in recent years due to the desirability of increasing safety, problems in acquiring right-of-way for overhead lines, higher elevations of power lines required in growing metropolitan areas, and growing demands for electrical energy. Compressed gas insulated transmission lines typically comprise a hollow sheath, a conductor within the sheath, a plurality of solid insulating spacers which support the conductor, and a compressed gas such as sulfur hexafluoride or the like in the sheath to insulate the conductor from the sheath. It is also known to provide a particle trap for compressed gas insulated transmission line as disclosed in the patent to Trump, U.S. Pat. No. 3,515,939. The particle trap of Trump is used to precipitate out of the insulating gas, particles of foreign matter which could adversely affect the breakdown voltage of the dielectric gas.
The present invention is directed to both vertically-oriented as well as horizontally-oriented gas insulated transmission lines. Each orientation requires different approaches to controlling particle contamination. In horizontally-oriented transmission lines, localized low-electric-field regions are provided adjacent a bottom portion of the outer sheath. Contaminating particles, whether conductive or semiconductive, are acted upon by electric fields internal to the transmission line, as well as by gravity, which cause migration of the particles to the low field regions where they are trapped or inactivated. In vertically-oriented transmission lines, particles fall downwardly through the transmission line, and may come to rest on generally horizontally-oriented insulator supports which extend between the energized central conductor and the outer sheath.
U.S. Pat. No. 4,096,345 issued June 20, 1978 to George A. Kemeny, describes a prior art conical insulator suspended between the inner conductor and outer sheath to prevent particles from travelling great distances through the transmission line. The prior art insulator arrangement did not provide a lowered electric field region. As an improvement over that arrangement, the Kemeny patent provided a more effective particle trapping system which had, in addition to a deflecting member, a low field region positioned in the outer sheath. In this arrangement, as in U.S. Pat. No. 3,515,939 issued to Trump, particles directed into regions of lowered field intensity became inactivated, reducing the likelihood that breakdown would be initiated, or that trapped particles would be dislodged during voltage surge conditions.
The arrangement described in the Kemeny patent requires outstruck annular channels to be formed in the sheath adjacent each particle trap. From fabrication, stocking and assembling standpoints, a continuously smooth outer sheath would be preferred. Also, the conical support insulator utilized by Kemeny to deflect contaminating particles has a substantial thickness, is attached at either end to inner conductor and sheath, and must be positioned at a substantial mimimal angle to the inner conductor. As a result, the deflector of Kemeny has a significant field along its surface.
While prior art particle traps offer generally satisfactory dielectric properties, improvements in field shaping with reductions in voltage stresses on particle trap surfaces is always desirable.
Accordingly, it is an object of the present invention to provide a particle trap system having a separate deflector shield which is suitable for use on both horizontally and vertically-oriented gas insulated transmission systems.
Another object of the present invention is to provide a particle trap system which exhibits greater dielectric strength, having reduced voltage stress levels on its surfaces.
It is a further object of the present invention to provide an inexpensive, easy-to-fabricate, non-load-bearing (and therefore thin) deflector member having a lowered electric field along its surface, and which can be mounted at reduced angles to the inner conductor.
It is another object of the present invention to provide an improved particle trap assembly which is compatible with smooth-walled outer sheaths.